Use of the lotus effect in process engineering

ABSTRACT

The invention provides an evaporator having a heatable heat exchange surface, which has a self-cleaning microstructured surface with elevations and depressions.  
     The microstructured surface of the heat exchange surface may be produced by powder coating. The invention additionally provides a process for the evaporative concentration of solutions in an evaporator having a self-cleaning microstructured heat exchange surface, the solution for evaporative concentration preferably comprising viscous or solid constituents in solution, emulsion or suspension. Using the evaporators of the invention, caking of the viscous or solid constituents on the heat exchange surfaces during the evaporative concentration of the solutions is prevented. As a result, such solutions may be concentrated to a solvent content of &lt;5% by weight.

[0001] The invention relates to evaporators having a microstructuredsurface and to processes for the evaporative concentration of solutions.

[0002] In process engineering, mixtures of substances including viscousor solid constituents are frequently separated using an evaporator.Examples are the concentration of polymer solutions, the concentrationof solutions laden with salts or resins, the workup of column bottomproducts, contaminated substances or sludges, and, in the food segment,the separation of solvents down into the ppm range.

[0003] A problem which arises with these mixtures of substances is thattheir highly viscous evaporation residues cake on the heat exchangesurfaces, thereby increasing the likelihood of clogging in theevaporator. Moreover, the deposits and incrustations which formdrastically reduce the heat transfer coefficient.

[0004] In order to effect virtually complete evaporative concentrationof suspensions or solutions which are viscous and tend to stick, it hashitherto been necessary to ensure, by applying mechanical force, thatthe heat exchange surfaces are kept free from attachments andincrustations. This is normally achieved by installing rotating wipersor scrapers into the evaporators, which keep the heat exchange surfacesclear. Examples of evaporator designs of this kind are the evaporatorswith anchor stirrer and evaporators with paddle stirrer shown on page613 of Klaus Sattler, Thermische Trennverfahren [Thermal SeparationProcesses], Wiley-VCH Weinheim, Berlin, New York, 2nd Edition 1995. Todate, where the solution to be concentrated by evaporation was to attaina pastelike or even solid state, the only suitable machines have beenscrew machines. However, these are very expensive and their heattransfer coefficient is poor.

[0005] It is an object of the present invention to provide alternativesto the prior art evaporators for the evaporative concentration ofsolutions comprising viscous or solid constituents with a tendency tostick. It is a particular object of the present invention to provide aheat exchanger which is inexpensive to produce and in which the heatexchange surfaces are kept clear from attachments and incrustationswithout applying mechanical force.

[0006] We have found that this object is achieved by means of apparatusand apparatus parts for chemical plant construction which have aself-cleaning microstructured surface with elevations and depressions.

[0007] Preferred apparatus and apparatus parts which may be furnishedwith a self-cleaning microstructured surface are internal apparatus,vessel and reactor walls, discharge devices, fittings, circuit systems,evaporators, filters, centrifuges, columns, dryers, internals, packings,and mixing elements.

[0008] Particularly preferred apparatus of the invention comprisesevaporators having a heatable heat exchange surface, wherein the heatexchange surface has a self-cleaning microstructured surface withelevations and depressions.

[0009] By apparatus and apparatus parts are meant:

[0010] Internal apparatus, vessel and reactor walls, discharge devices,fittings, circuit systems, evaporators, pumps, filters, compressors,centrifuges, columns, dryers, size reduction machines, internals,packings, and mixing elements, preferably internal apparatus, vessel andreactor walls, discharge devices, fittings, circuit systems,evaporators, filters, centrifuges, columns, dryers, internals, packings,and mixing elements, and, with very particular preference, evaporators.

[0011] More specifically, said apparatus and apparatus parts may bedescribed as follows:

[0012] Vessels comprise, for example, reservoir or collecting vesselssuch as troughs, silos, tanks, vats, drums or gasometers, for example.

[0013] Apparatus and reactors comprise liquid, gas/liquid,liquid/liquid, solid/liquid or gas/solid reactors, and gas reactors,which are embodied, for example, in stirred, jet loop and jet nozzlereactors, jet pumps, delay time cells, static mixers, stirred columns,tube reactors, cylinder stirrers, bubble columns, jet scrubbers andventuri scrubbers, fixed-bed reactors, reaction columns, evaporators,rotary disk reactors, extraction columns, kneading and mixing reactorsand extruders, mills, belt reactors, rotary tubes, or circulatingfluidized beds.

[0014] Discharge devices comprise, for example, discharge ports,discharge hoppers, discharge pipes, valves, discharge cocks or ejectordevices.

[0015] Fittings comprise, for example, stopcocks, valves, slide valves,bursting disks, nonreturn valves, or disks.

[0016] Pumps comprise, for example, centrifugal, toothed wheel, screwspindle, eccentric screw, rotary piston, reciprocating piston, membrane,screw trough or jet liquid pumps, and also reciprocating piston,reciprocating piston membrane, rotary piston, rotary slide valve, liquidseal, lobe or pump-fluid vacuum pumps.

[0017] Filter apparatus comprises, for example, fluid filters, fixed-bedfilters, gas filters, sieves or separators.

[0018] Compressors comprise, for example, reciprocating piston,reciprocating piston membrane, rotary piston, rotary slide valve, liquidseal, rotary, Roots, screw, jet or turbocompressors.

[0019] Centrifuges comprise, for example, screen-jacket or solid-jacketcentrifuges, preference being given to plate centrifuges, full-jacketscrew centrifuges (decanters), screen screw centrifuges and reciprocalpusher centrifuges.

[0020] Columns comprise vessels with exchange trays, preference beinggiven to bubble, valve or sieve trays. The columns may also have beenpacked with different packings, such as saddles, Raschig rings, orbeads.

[0021] Size reduction machines comprise, for example, crushers,preference being given to hammer, impact, roll or jaw crushers, ormills, preference being given to hammer, cage, pin, impact, tube, drum,ball, vibratory and roll mills.

[0022] Internals in reactors and vessels comprise, for example,thermocouple sheaths, flow disrupters, foam breakers, packings, spacers,centering devices, flange connections, static mixers, analyticalinstruments such as pH or IR probes, conductivity meters, levelmeasuring instruments, or foam probes.

[0023] The self-cleaning surfaces may be microstructured as described inWO 96/04123. The surface described therein has elevations anddepressions, the distance between the elevations being from 5 to 200 μm,preferably from 10 to 100 μm, the height of the elevations being from 5to 100 μm, preferably from 10 to 50 μm, and at least the elevationsbeing of a hydrophobic material. The water repellency of these surfacesis attributed to the fact that the water drops lie only on the peaks ofthe elevations and thus have only a small area of contact with thesurface. The water drop, occupying the smallest possible surface area,forms a bead and rolls off from the surface at the slightest vibration.The adhesion of solid particles to the surface is similarly reduced.These particles have a more or less great affinity for water, so thatthey are removed from the surface together with the drops which rolloff.

[0024] A self-cleaning microstructured surface in accordance with oneembodiment of the invention may be described as follows.

[0025]FIG. 1 shows the idealized representation of a section through aself-cleaning microstructured surface in accordance with one embodimentof the invention. The idealized microstructured heat exchange surface(1) has hemispherical elevations (2) of radius R, arranged with aspacing s, and depressions in between. The distance s between theelevations (2) is such that a liquid (3) hanging down between theelevations occupies a radius of curvature R*, and in the depressionsbetween the elevations (2) does not contact the heat exchange surface(1). Preferably, s <4R. In the vapor space (4), the vapor pressure Pv ofthe liquid (3) at the system temperature is established; in the case ofideal mixtures, the sum of the vapor pressures of the components. Thedownward-hanging curve of the liquid is subject to the sum of this vaporpressure pv plus the hydrostatic pressure P_(hy), i.e., in the case of ahorizontal heat exchange surface:

P_(v)+P_(liq)gh.

[0026] It is known that the vapor pressure over curved phase boundariesis greater than over planar phase boundaries. The vapor pressure over acurved surface is

p_(v)(R*)=P_(v) exp (2Φ_(AB) V_(liq)/R* ΘT),

[0027] where p_(v)(R*) is the vapor pressure over the phase boundarywith the radius of curvature R*, P_(v) is the vapor pressure over theplanar phase boundary, σ_(AB) is the surface tension between the liquidphase and the solid phase of the elevation (2), V_(liq) is the molarvolume of the liquid phase, R* is the radius of curvature of thedownward-hanging liquid, θ is the ideal gas constant, and T is thetemperature.

[0028] The structure of the surface, then, is such that R* is so smallthat at the anticipated film thicknesses of h the vapor pressurep_(v)(R*) always remains at least equal to the sum of P_(v)+P_(hy). Inthat case, the liquid (3) is unable to wet the surface.

[0029] Rather than by a hydrostatic pressure P_(hy), the pressureprevailing at the curve of the liquid may be increased by an additionalcentrifugal pressure, brought about by a centrifugal force. Such acentrifugal force acts, for example, in a coiled tube evaporator. Theabove relationships apply accordingly.

[0030] The nonwettability of surfaces can therefore be attributed to thevapor pressure increase in small drops. This effect may be intensifiedby heating the surface. In evaporators, the heat exchange surface isgenerally hotter than the liquid. In that case the liquid repellency issupported by the additional increase of the vapor pressure in theenclosed vapor bubble. If the system reaches nucleate boiling, thesurface is protected even more effectively against wetting. The liquidcurves become boil bubbles of radius R*.

[0031] Similar laws apply to systems additionally comprising inertgases. In that case, it is necessary to take account of the desorptionpressures of the soluble inert gases, as well as the vapor pressures.With dissolved gases, too, the desorption pressure is increasedanalogously by a curved phase boundary.

[0032] Real microstructured surfaces, depending on the nature of theirproduction, will generally have a geometry which deviates to a greateror lesser extent from the idealized geometry indicated in FIG. 1. Inparticular, the elevations (2) will not be exactly hemispherical andtheir radius R and distance s will vary to a greater or lesser extent.Moreover, the depressions lying between the elevations (2) need not beplanar. Preferably, however, the elevations will have an essentiallyrounded form and will have on average a radius R of from 5 to 100 μm anda distance s of from 5 to 200 μm.

[0033] The microstructured surface may be produced by powder coating ofadhesives and coating materials applied to the surface. This can be doneby, for example, blowing or powdering hydrophobic pigments, Teflonpowders, wax powders, polypropylene powders or similar particulatesubstances of appropriate particle size onto the surface wetted with thecoating material or adhesive. Preferably the powders have a narrowparticle size distribution. Microstructured surfaces may also beobtained by layer deposition from solutions, electrolytic deposition,galvanic techniques, etching techniques, or vapor deposition.

[0034] The particulate substance to be applied, and the polarity of themicrostructured surface, are chosen as a function of the solvent to beevaporated. In the case of aqueous, aqueous-organic or polar organicsolvents, the microstructured surface will have hydrophobic properties.For separation of nonpolar solvents, however, it is also possible toequip the evaporator with a hydrophilic microstructured surface.

[0035] The effect of the microstructure of the heat exchange surface isthat both solvents and solid particles or viscous residues are almostcompletely unable to adhere to it. The surface is unwettable. The resultof this is that a subsequent flow of liquid picks up, and removes, thesolid or viscous constituents occupying the surface.

[0036] In principle, all common types of evaporator may be equipped witha microstructured surface. Particularly advantageous results, however,are achieved with those types of evaporator in which there is acontinuous flow of liquid over the heat exchange surfaces. Preference isgiven to forced circulation evaporators, flash evaporators, and coiledtube evaporators. In a forced circulation evaporator, the liquidintended for evaporative concentration is conveyed over the surface bymeans of a pump. The coiled tube evaporator has an evaporator tube whichis curved in the form of a coil. This type of evaporator is described inDE-C 2 719 968. By virtue of a high gas flow rate (preferably >20 m/s),the liquid is pressed against the tube wall, forcing the development ofan annular flow. The superimposed centrifugal force ensures thedevelopment of a secondary flow, which improves the heat transfer. Inthe case of a microstructured surface coiled tube evaporator inaccordance with a preferred embodiment of the invention, surprisingly,the evaporative concentration of solutions containing viscous residuesis not accompanied by caking of the residues, and at the same time ahigh heat transfer coefficient is realized. These advantages areachieved similarly in all evaporator tubes with a microstructuredsurface in which an annular flow is able to develop. For example, theevaporator used may be a straight tube, with an annular flow beingforced by means of a high gas speed.

[0037] The present invention additionally provides a process for theevaporative concentration of solutions in an evaporator having aself-cleaning microstructured heat exchange surface. The solution forevaporative concentration preferably comprises viscous or solidconstituents. These constituents may be present in suspension, emulsionor solution in the solution for evaporative concentration. Examples areaqueous or aqueous-organic solutions of inorganic salts, such as aqueousbutynediol solutions containing catalyst. If a hydrophilic surface isemployed, it is also possible to carry out evaporative concentration oforganic solutions, an example being dehydrocholesterol acetate inxylene.

[0038] With the process of the invention it is possible to carry outevaporative concentration of solutions down to a solvent content ofgenerally <10% by weight, preferably <5% by weight, with particularpreference <2% by weight.

[0039] The invention is illustrated by the following examples.

EXAMPLES Experimental apparatus

[0040] The experiments were conducted in the experimental installationdepicted in FIG. 2. The solution containing viscous residues was pumpedfrom the reservoir vessel (1), which was located on a balance, into thepreheater (3) with the aid of the gear pump (2). In the preheater (3),the solution was heated under pressure and then released through thecontrol valve (4) into the evaporator (5) of the invention. The volumeflows and the pressure were set in such a way that an annular flowdevelops in the evaporator. The evaporator comprised a coiled glass tubehaving an internal diameter of 6 mm. 20 coils having a diameter of 70 mmand a pitch of approximately 20° were installed. The glass coils wereequipped with a microstructured surface produced by applying UV-curablecoating material (Laromer PO 84 F), coating it with Teflon powder havingan average particle size of 7 μm (Dyneon TF 9205 PTFE), and then curingthe system. The discharge from the evaporator was passed into the heatedphase separator (6), in which liquid phase and vapor phase wereseparated. The residue (liquid phase) was collected in the vessel (7).The vapor was condensed in the condenser (8) and collected in the vessel(11). The apparatus was operated under atmospheric pressure using thevacuum pump (10) and the control valve (9).

Example 1

[0041] Evaporative concentration of an aqueous butynediol solution

[0042] A 50% strength aqueous solution of butynediol containingapproximately 5% by weight Raney nickel (catalyst) was subjected toevaporative concentration in the apparatus described above. For thispurpose, 13.8 g/h of the solution were preheated to 150° C. at 10 barand were released into the evaporator at 20 mbar. The salt-free aqueousbutynediol solution was obtained as the distillate. The residue was aviscous crystal slurry having a residual water content of approximately1% by weight. With the evaporator having the microstructured surface, noincrustation of the evaporator surface was observed.

[0043] In a comparative experiment with a conventional evaporatorcorresponding to the geometries described above, the surface becameoccupied. The only way to compensate this was by reducing the degree ofconcentration, entailing a reduction in the product yield.

Example 2

[0044] Evaporative concentration of a salt solution

[0045] A 10% strength aqueous NaCl solution was subjected to evaporativeconcentration in the apparatus described above. For this purpose, 20 g/hof the solution were preheated to 150° C. at 10 bar and released intothe evaporator at 20 mbar. In this way, the solution was concentrated togive a residue comprising a viscous crystal slurry having a residualwater content of approximately 1% by weight. The crystals precipitatingon the evaporator surface were continually washed away by a subsequentflow of water. No incrustation of the evaporator surface was observed,even with a high degree of concentration. Salt-free water was obtainedas the distillate.

We claim:
 1. An apparatus or part thereof for chemical plantconstruction, having a self-cleaning microstructured surface withelevations and depressions.
 2. An apparatus or part thereof as claimedin claim 1 , selected from the group consisting of internal apparatus,vessel and reactor walls, discharge devices, fittings, circuit systems,evaporators, filters, centrifuges, columns, dryers, internals, packings,and mixing elements.
 3. An apparatus as claimed in claim 2 in the formof an evaporator having a heatable heat exchange surface, wherein theheat exchange surface has a self-cleaning microstructured surface withelevations and depressions.
 4. An evaporator as claimed in claim 3 inthe form of a forced circulation, flash or coiled tube evaporator.
 5. Anevaporator as claimed in claim 3 in the form of an evaporator tube inwhich an annular flow is able to develop.
 6. An evaporator as claimed inclaim 3 , wherein the microstructured surface of the heat exchangesurface is produced by powder coating.
 7. A process for the evaporativeconcentration of solutions in an evaporator having a self-cleaningmicrostructured heat exchange surface as defined in claim 3 .
 8. Aprocess as claimed in claim 7 , wherein the solution for evaporativeconcentration comprises viscous or solid constituents in solution,emulsion or suspension.
 9. A process as claimed in claim 7 , wherein thesolution for evaporative concentration is an aqueous or aqueous-organicsalt solution.
 10. A process as claimed in claim 8 , wherein evaporativeconcentration is conducted to a solvent content of <5% by weight.